1. Field of the Invention
The present invention relates generally to cylinder formers in papermaking machines and other industrial applications such as fiber cement (FC) production and more specifically to an extended couch nip with a pressure shoe in the forming section of a cylinder mould that replaces the traditional couch roll to more effectively transmit torque from a making fabric to a cylinder mould or sieve.
2. Background of the Invention
Typically, during the process for making paper products such as but not limited to paper, paper board and carton board, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a papermaking machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is sequentially directed in a serpentine path around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
Presently, there are numerous ways of forming a continuous sheet of paper, paper board, and carton board. For example, continuous paper sheets, can be formed using a number of separate forming sections. The capital cost required to install a multi-fourdrinier papermaking machine, however, is high and sometimes the change is not feasible because of the total capital required. Additionally, larger space requirements are required for this type of papermaking machine. Another factor to consider in choosing which forming process to use may be the weight of the board to be produced or the properties of the board to be developed. Accordingly, in certain applications, the use of a cylinder mould in formation is desirable.
The principle of sheet formation using a cylinder former is depicted in FIG. 1 and is as follows. A horizontal cylinder (cylinder mould or sieve) 14 having a woven fabric sleeve is arranged to rotate approximately three quarters submerged in a container (vat) 22 of paper or other stock 20 so that a small arc of the cylinder's circumference is above stock level. Stock in this case is defined as a fibrous suspension and water. The fiber can be cellulose, synthetic or natural. Other additives such as inorganic particles necessary for development of product properties may also be present. Water 21 associated with the fibrous suspension drains through the woven fabric sleeve, resulting in a layer of fibers deposited on the surface of the fabric sleeve. Drainage takes place because of a difference in the water levels between the stock in the vat 22 and the backwater 23 inside the mould 14. The difference is known as the making head.
A moving fabric or “making fabric” 16 is then pressed by means of a couch roll 12 into contact with the cylinder mould 14 at approximately its top position. By doing this, a layer of fibers (fibrous web or fibrous suspension) that has formed on the fabric sleeve is transferred or couched to the making fabric 16 and moves away from the fabric sleeve with the fabric 16. The fibrous layer 18 formed on the fabric sleeve is transferred to the making fabric 16 upon contact by virtue of the fact that the making fabric 16 is less porous and smoother than the fabric sleeve, as a consequence of which atmospheric pressure facilitates the transfer. As the couch roll 12 compresses the making fabric 16 against the fabric sleeve on the cylinder mould or sieve 14, the making fabric 16 is performing multiple tasks. The fabric 16 is picking the wet fibrous web layer 18 off the sleeve surface on the cylinder mould 14, The making fabric 16 also acts as a drive belt for the entire forming/press section. Finally, the making fabric partially dewaters the fibrous web layer(s) by providing void volume or receptacles within the fabric for the water to go that is pressed out or removed by vacuum from the fibrous layer(s). Since a cylinder mould 14 is typically not connected to a driving means, the making fabric 16 is the source of rotation for the cylinder mould 14. Once the fibrous web 18 has been transferred to the making fabric 16, the sleeves of the cylinder mould 14 are washed by sprays and any fibrous material not transferred to the making fabric 16 enters into the fiber stock reservoir 20 for use in forming a new layer 18.
As depicted in FIG. 2, a number of these units can be placed in series resulting in a multi-cylinder machine. In a multi-cylinder machine, a multi-ply web or sheet is produced continuously. Each forming unit typically has its own supply of stock and a method of removing the drainage water from its interior so that, in effect, each cylinder mould is a separate web forming unit in itself. As the making fabric passes through successive units, additional layers of fibers are transferred or couched to the fibrous web that is already adhered to the making fabric.
Cylinder mould formation of the type described above may also be used in fiber cement (FC) board production. In the FC industry, cylinder mould formation is known as the “Hatscheck” process. In this process, a cementitious slurry is initially formed from water, cellulose fiber, silica, cement and other additives selected to impart particular properties to the product according to its intended application. Similarly to papermaking, a sieve cylinder or mould is immersed into a vat that contains the slurry. The cylinder rotates as it is progressively driven by the bottom run of a making fabric. As the making fabric passes over the cylinder and contacts the mesh screen of the cylinder, the layer of fiber formed on the screen is transferred to the making fabric. As in papermaking, a number of these units can be placed in series resulting in a multi-cylinder machine. This process can be applied to make numerous types of FC products used in the construction industry such as but not limited to FC board and FC pipe.
Various types of cylinder moulds and vat arrangements currently exist. In this regard, one typical cylinder mould is constructed around a cast iron core upon which is secured supporting spokes known as spiders. The spiders support concentric rims, the outside peripherals of which are grooved in order to carry rods that are approximately 1 centimeter in diameter and approximately 3.5 centimeters apart, parallel with the axis of the central shaft. A continuous wire is wound around the cylinder. This skeleton is traditionally covered with a stainless steel wire, typically in the 30-mesh to 50-mesh range. Synthetic sleeves, often made of polyethylene (PE), polyvinylidene fluoride (KYNAR®) and polyphenylene sulfide (RYTON®, PPS), etc., are typically woven and installed onto the cylinder mould or sieve in order to increase the fiber support as well as control formation by controlling drainage. The properties and weave patterns of the synthetic sleeves, however, can make it difficult for the making fabric to drive the cylinder mould due to a reduced friction between the mould and the fabric. The ability of the fabric to transmit torque to the mould, which results in rotation of the mould, is affected by tension (pressure from the couch roll) and the amount of contact between the couch roll and the mould, both of which affect the amount of friction between the two. Therefore, an improved means is needed to increase friction and effectively transfer torque from the making fabric to the cylinder mould in order to drive all the cylinder moulds.
Although, as previously stated, various types of cylinder moulds and vat arrangements exist, they will not be discussed in detail since the present invention can be equally applied to the various cylinder moulds and vat arrangements.
Prior devices have not been developed to increase the ability of the making fabric to drive a cylinder mould or sieve in a cylinder former. For example, U.S. Pat. No. 5,695,612 discloses a prepress for a paper web in a papermaking machine that uses a pressure shoe in conjunction with a backing element to apply a pressure to a paper web. The web passes between the load shoe and the backing element and is preferably positioned between two wires or fabrics. A medium is used to apply pressure to the load shoe to remove water from the paper web. The medium can also be passed through channels in the load shoe to lubricate the front surface of the load shoe's web plate. Here, the load shoe is not used in conjunction with a cylinder mould or sieve. The load shoe's function is not to increase the friction between a making fabric and a cylinder mould thereby increasing the making fabric's ability to drive a cylinder mould or sieve in a cylinder former.
Similarly, PCT Publication No. WO 01/51703 discloses a method and device for prepressing a paper web during web formation. A web of paper or a paper board is sandwiched between a pair of forming wires. In various embodiments, the sandwich of forming wires and paper web then passes through one or more pressure nips where the pressure nips may be one or more roll nips or an extended nip press which has a pressure shoe to press the web along a portion of the length of the web. Again, the pressure shoe in this instance does not increase the friction between a making fabric and a cylinder mould thereby enhancing the ability of the fabric to drive the cylinder mould in a cylinder former.
U.S. Pat. No. 4,308,097 discloses a paper web former for producing a paper web of fibrous suspension on a wire. The former comprises a convex shoe with an opening through which the pulp suspension exits onto a sliding surface of the shoe. The configuration that uses this former still uses couching rolls to press out the webs and couch them to a conveying (making) fabric. The former does not replace the couch roll and is not in a “nipping” relationship (where the shoe in conjunction with a backing element applies pressure to the fibrous web) with a cylinder mould.
In U.S. Pat. No. 4,880,500, a papermaking machine is modified by replacing a conventional rotatable couch roll with a stationary couching device. The stationary couching device has a member with a convexly curved and slotted upper surface on which the web slides. The convexly curved couching device is not in a “nipping” relationship with a cylindrical mould so the device is not used to increase friction and transfer torque from a making fabric to a cylinder mould in order to rotate the mould.
Lastly, U.S. Pat. No. 4,919,760 discloses a web former for a paper machine having a top wire and a lower wire. A forming shoe is fitted inside the lower wire loop and after a first forming roll in the web run direction, and guides the twin-wire dewatering zone part. The forming shoe has a convexly curved deck for guiding the lower wire loop. The forming shoe's placement in the paper machine facilitates water removal and water collection from the web without suction. Instead, water is collected and removed on the basis of kinetic energy, and partially on the basis of gravity. The forming shoe having a convexly curved deck is not in a “nipping” relationship with a cylindrical mould. Therefore, the device is not used to increase friction and transfer torque from a pick-up fabric to a cylinder mould in order to rotate the mould.
Accordingly, a need exists for an extended couch nip having a pressure shoe for use on a cylinder former, that increases the nip to a greater area of the making fabric so as to improve the ability of the fabric to drive the cylinder mould(s) or sieve(s) by increasing friction between the two.